The present invention relates to a process for removing undesirable impurities from 3-(2xe2x80x2-acetoxyethyl)dihydro-2(3H) furanone of the formula I 
3-(2xe2x80x2-Acetoxyethyl)dihydro-2(3H)furanone is a starting material for preparing methyl tetrahydropyran-4-carboxylate which for its part is an intermediate in the preparation of crop protection agents.
3-(2xe2x80x2-Acetoxyethyl)dihydro-2(3H)furanone is prepared, for example, by processes known per se and described, for example, in U.S. Pat. No. 5,350,863, starting from methyl acetoacetate and ethylene oxide. Another process variant from which 3-(2xe2x80x2-acetoxyethyl)dihydro-2(3H)furanone is obtainable is described in Dokl. Akad. Nauk SSSR 27, 956-959 (1940) and U.S. Pat. No. 5,283,326.
In both variants, the desired 3-(2xe2x80x2-acetoxyethyl)dihydro-2(3H)furanone is formed as a mixture with a number of undesirable byproducts, in particular the isomeric dihydro-3-(2-methyl-1,3-dioxolan-2-yl)-2(3H)-furanone II 
If I which is contaminated in this manner is reacted in a continuous gas phase reaction with methanol in the presence of acidic catalysts to give methyl tetrahydropyran-4-carboxylate, the yields are significantly lower and the operating life of the catalyst is significantly shorter than when a product of high purity without byproducts is employed. This is probably due to the fact that most byproducts contain labile acetal groups which decompose on the surface of the catalyst to give oligomeric and polymeric products.
It is not possible to remove the undesirable byproducts, in particular the isomeric dihydro-3-(2-methyl-1,3-dioxolan-2-yl)-2(3H)furanone II adequately by simple batchwise distillation or rectification. In addition, during the batchwise distillation or rectification of 3-(2xe2x80x2-acetoxyethyl)dihydro-2(3H)furanone I containing the undesirable impurities, other low-boilers and intermediate boilers which are difficult to remove are formed, which recontaminate the purified I obtained in the distillate.
It is an object of the present invention to provide a process for removing undesirable byproducts, in particular II, and also newly formed low-boilers and intermediate boilers from mixtures with I.
We have found that this object is achieved by a process for removing impurities from 3-(2xe2x80x2-acetoxyethyl)dihydro-2(3H)furanone (I), where the 3-(2xe2x80x2-acetoxyethyl)dihydro-2(3H)furanone containing the undesirable impurities is initially prepared in a manner known per se by acetylation of 3-(2xe2x80x2-hydroxyethyl)dihydro-2(3H)furanone and the resulting product is subsequently subjected to a distillation or rectification, which comprises carrying out the distillation or rectification in a plurality of steps, where high-boiling impurities are separated off in a first step, the product which is drawn off via the top is subsequently subjected to a further step in which low-boilers and intermediate boilers are drawn off via the top and the desired pure I is obtained as bottom product.
Surprisingly, the process according to the invention affords a significantly higher degree of purity of the desired 3-(2xe2x80x2-acetoxyethyl)dihydro-2(3H)furanone (I) than in the customary sequence in the distillation or rectification where low-boilers and intermediate boilers are removed before the high-boilers and the product of value is drawn off at the top of the column. The reason for this is that, during the removal of the high-boilers, low-boilers and intermediate boilers are formed which can then not be removed in the customary sequence of the steps and which reduce the purity of the top product.
Preferred embodiments of the process according to the invention are shown in the subclaims and the description below.
The process according to the invention starts with the mixture of 3-(2xe2x80x2-acetoxyethyl)dihydro-2(3H)furanone (hereinbelow referred to as I) and undesirable impurities which is obtainable by known processes described in the introduction.
This mixture is generally prepared by acetylation of 3-(2xe2x80x2-hydroxyethyl)dihydro-2(3H)furanone. The preferred acetylating agent is acetic anhydride or acetic acid; however, in principle, all acetylating agents which are known to the person skilled in the art for corresponding acetylations are suitable. In general, the acetylation is carried out at temperatures in the range from 40xc2x0 C. to 200xc2x0 C., preferably from 60 to 140xc2x0 C., over a period of from 0.5 to 10 h, preferably from 0.8 to 5 h and in particular from 1 to 3 h.
To ensure complete acetylation, the acetylating agent is generally employed in a molar excess of from 5 to 50%, preferably from 5 to 20%, based on the 3-(2xe2x80x2-hydroxyethyl)dihydro-2(3H)furanone present in the mixture.
In a first preferred embodiment of the process according to the invention, the acetic acid formed as coproduct during the acetylation is firstly distilled off. By adding a suitable organic solvent and water, a phase separation is then carried out and the organic phase is subsequently extracted. Suitable organic solvents are in particular aromatic hydrocarbons and particularly alkylated benzene derivatives such as xylene or toluene. The organic extracts are combined and the solvent is then removed by a preliminary distillation or rectification step in at least one column, before the multistep distillation of claim 1 is carried out.
In another preferred embodiment of the process according to the invention, the product formed during the acetylation is treated with strong mineral acids before it is subjected to the rectification or distillation. Preferred mineral acids are hydrochloric acid and particularly preferably sulfuric acid. The latter is preferably employed in a strength of at least 80% and especially in the form of concentrated sulfuric acid.
The amount of acid used here can be varied within a wide range; in some cases, it has been found advantageous for the molar ratio of strong mineral acid, based on the amount of acetylating agent employed in the previous step, to be in the range of from 1:20 to 1:3, in particular from 1:15 to 1:7 and especially from 1:8 to 1:12.
During the treatment with the mineral acid, the temperature is generally in a range from 10 to 80xc2x0 C., preferably from 20 to 60xc2x0 C. and in particular from 30 to 50xc2x0 C. The duration of the treatment is generally in the range from 0.3 to 10 h, in particular from 1 to 5 h, but can, in principle, be varied within wide limits.
After the treatment with strong mineral acids, the decomposition products of the undesirable impurities, in particular of dihydro-3-(2-methyl-1,3-dioxolan-2-yl)-2(3H)-furanone II, can be removed in a simple manner. Particularly preferably, after the treatment with the mineral acid, the mineral acid is initially neutralized by addition of a base and the acetic acid formed as coproduct during the acetylation is subsequently, if appropriate, removed as described in the first preferred embodiment. This is then followed by the multistep distillation or rectification of claim 1.
The purity of I after the process according to the invention is generally at least 98% by weight, preferably at least 98.5% by weight and particularly preferably at least 99% by weight.
The I obtainable by the process according to the invention can be reacted in a known manner in high yield and with good catalyst operating life to give methyl tetrahydropyran-4-carboxylate. This is an important intermediate in the preparation of crop protection agents.